1. Field of the Invention
The present invention relates to a magnetic resonance imaging (MRI) system for applying a gradient magnetic field and an RF pulse to an object to be examined which is placed in a static magnetic field so as to excite magnetic resonance at a specific portion of the object, and acquiring magnetic resonance (MR) echo signals excited by the magnetic resonance, thereby imaging the specific portion by a predetermined image reconstruction method using data based on the acquired MR echo signals and, more particularly, an improvement of multi-echo sequence in a spin echo method (to be referred to as an SE method hereinafter) using 90.degree.-180.degree. series RF pulses.
2. Description of the Related Art
In a general medical MRI system, a gradient magnetic field and an RF pulse are applied to an object to be examined which is placed in a static magnetic field in accordance with a predetermined sequence for magnetic resonance excitation/MR data acquisition so as to cause an MR phenomenon at a specific portion of the object, and an MR signal excited by the MR phenomenon is detected. In addition, according to the system, data processing for imaging which includes image reconstruction is performed for MR data acquired in this manner so as to image anatomical information or quality information of the specific portion of the object.
An MRI system of this type generally comprises a static magnetic field generator, X-axis, Y-axis, and Z-axis gradient magnetic field generators, an RF transmitter, and an RF receiver. The X-axis, Y-axis, and Z-axis gradient magnetic field generators and the RF transmitter are driven in accordance with a predetermined sequence so as to generate X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz and an RF pulse in accordance with a predetermined sequence pattern. As a result, MR is excited to generate an MR signal, and the MR signal is received by the receiver. Predetermined image processing including image reconstruction processing is performed for the received MR data. In this manner, a tomographic image of a certain slice portion of an object to be examined is generated and displayed on a monitor.
In the sequence for magnetic resonance excitation/MR data acquisition, the X-axis, Y-axis, and Z-axis gradient magnetic fields Gx, Gy, and Gz are respectively used as, e.g., a read gradient magnetic field Gr, an encode gradient magnetic field Ge, and a slicing gradient magnetic field Gs.
One of the conventional MR methods widely used in such a system is an imaging method employing the sequence of the SE method which uses 90.degree.-180.degree. series RF pulses. According to the sequence of the SE method, data acquisition can be performed by a multi-echo sequence in which a plurality of MR echoes are sequentially generated upon one MR excitation and the respective data are sequentially acquired. This SE method is often used for MR data acquisition using the multi-echo sequence.
The sequence of such a conventional SE method will be described below with reference to FIG. 1. FIG. 1 shows a sequence in one encode step.
A slicing gradient magnetic magnetic field Gs and a 90.degree. selective excitation pulse as an RF magnetic field are applied to an object to be examined so as to excite a specific slice of the object (to flip the magnetization vector (to be referred to as "nuclear magnetization" hereinafter) of the nuclear spin of a specific atomic nucleus in the slice through 90.degree.). Thereafter, an encode gradient magnetic field Ge having an amplitude corresponding to the encode step is applied to the object, and a 180.degree. pulse as an RF magnetic field is applied to the object so as to invert the nuclear magnetization, thereby rephasing and refocusing the rotational phase of the nuclear magnetization (which has been dephased and dispersed upon application of the 90.degree. pulse). In addition, a read gradient magnetic field Gr is applied to the object to generate an MR echo signal whose peak appears after a TE time (echo time) from the peak of the 90.degree. pulse. While the read gradient magnetic field Gr is applied to the object, the MR echo signal is acquired.
The above-described sequence is repeated while the amplitude of the encode gradient magnetic field Ge, which is applied between application of 90.degree. and 180.degree. pulses, is changed by a predetermined value in every encode step. When the second and subsequent multi-echo signal data are to be acquired, an operation of applying a 180.degree. pulse to the object after a time TE/2 from the peak of the immediately preceding echo signal and applying a read gradient magnetic field Gr to the object is repeated, thereby sequentially acquiring echo signals each having a peak appearing after a time TE/2 from the peak of each 180.degree. pulse.
In the sequence of the SE method, in order to minimize the influences of inhomogeneity of a static magnetic field, time t=0 at which a 90.degree. pulse is applied, time t=T.pi. at which a 180.degree. pulse is applied, and time t=TE at which the peak of a spin echo signal appears must satisfy the following equation: EQU T.pi.=TE/2
In this case, preferably, the earliest timing at which echo signal acquisition can be started comes at a point A in FIG. 1 after application of a 180.degree. pulse having a pulse time width TW, at which the leading edge of the read gradient magnetic field Gr is stabilized after the trailing edge of the slicing gradient magnetic field Gs. If data acquisition is to be performed in a symmetrical manner with respect to the echo peak at time t=TE, an echo signal acquisition time Taq is limited as follows: EQU TE-Tw-2.alpha..gtoreq.Taq
where is either the fall time of a slicing gradient magnetic field Gs or the rise time of a read gradient magnetic field Gr. If the resolution remains the same, the upper limit of the time Taq is determined by TE, Tw, and .alpha.. In addition, since the strength of a gradient magnetic field cannot be much decreased and ##EQU1## the signal-to-noise (S/N) ratio cannot be increased.
As described above, according to the conventional system, a time 1/2 the echo time (the time interval between the peak of a 90.degree. pulse and the peak of an echo signal) TE is set to be a time T.pi., and a 180.degree. pulse is applied. That is, EQU TE/2=T.pi.
Therefore, the upper limit of the echo signal acquisition time Taq is determined as TE-Tw-2.alpha..gtoreq.Taq, and an increase in S/N ratio is undesirably limited when the resolution and the TE time remain the same.
In recent years, however, techniques for obtaining homogeneity of a static magnetic field has progressed in MRI systems, and hence inhomogeneity of a static magnetic field can be reduced to such an extent that no problem is posed in practical use.